Barnier hits back at David Davis after the Brexit secretary accuses him of being "discourteous."

He says a transition deal is "not a given" if Britain remains inflexible.

Davis accuses Barnier and the EU of wanting to "have it both ways."

LONDON — Border checks between Northern Ireland and the Republic of Ireland are now "unavoidable" following Theresa May's decision to leave the customs union and single market, the EU's chief negotiator has insisted.

Speaking at the end of the latest round of Brexit negotiations, Michel Barnier said the UK's commitment to a hard Brexit meant that new border checks would have to be installed.

He said the failure to tackle the issue could mean that the EU may refuse to agree a Brexit transition deal with the UK.

"Once again it is important to tell the truth," Barnier told journalists in Brussels.

"A UK decision to leave the single market and to leave the customs union would make border checks unavoidable."

Barnier said the UK has to accept the "consequences" of its decisions.

He said that the EU would draft a legal text declaring that Northern Ireland will remain in the customs union after Brexit as one way of avoiding a hard border.

However, such a solution has already been ruled out by May's government, making a new hard border with Ireland almost inevitable.

The row is the latest ramping up of tensions between the UK and EU during the opening of negotiations on Britain's planned two year Brexit transition period, after it leaves the EU in March 2019.

The UK is seeking to negotiate a 'standstill' transition during which Britain will effectively remain within the single market and customs union in order to allow businesses to prepare for a complete break with the EU.

However, Barnier said that a deal could not be guaranteed, if Britain remains inflexible.

"The UK insisted that we should reach an agreement in March on transitional period. However, at the same time, our partners set out a number of disagreements which I see as substantial," he said.

"To be frank, if these disagreements persist, transition is not a given."

Brexit Secretary David Davis later hit back at Barnier, saying it was "surprising" to hear his comments, and accused the EU of trying to "have it both ways" over the proposed transition.

"Given the intense work that has taken place this week it is surprising to hear that Michel Barnier is unclear on the UK's position in relation to the implementation period. As I set out in a speech two weeks ago, we are seeking a time-limited period that maintains access to each other's markets on existing terms," Davis said.

"However for any such period to work both sides will need a way to resolve disputes in the unlikely event that they occur.

"But there is a fundamental contradiction in the approach the Commission is taking. Today they acknowledged that a way to resolve disputes and infringements is needed. Yet at the same time, they dismissed the UK's push for reasonable safeguards to ensure our interests are protected. It is not possible to have it both ways.

"We must not lose sight of the ultimate aim here — to build a new comprehensive partnership between the UK and the EU that sees us stay as the closest of friends and allies. It is in that spirit that the UK continues to approach negotiations with the Commission."

Watch Barnier on an "unavoidable" hard Brexit border

Labour said that Barnier's intervention should be a "wakeup call" for the government.

"This should be a wakeup call for the Prime Minister," Shadow Brexit Secretary Keir Starmer said.

"As Labour has said for months, it's vital the Government secures sensible transitional arrangements to prevent a cliff-edge for our economy. Yet with just a few weeks until the March deadline, it's clear the Government is no closer to reaching an agreement."

Davis and Barnier in Brexit war of words

Wiktor Dabkowski/DPA/PA Images

The row follows comments by the Brexit secretary David Davis on Thursday in which he accused the EU commission of "discourteous" behaviour by publishing EU guidelines which suggested the UK would be punished with sanctions if it failed to stick to the terms of any Brexit transition deal.

The document suggested Britain's access to the single market could be restricted if Britain failed to live up to its promises in negotiations.

Under the plans released by the European Commission, Brussels would be able to "suspend certain benefits" of the internal market for the UK without the UK retaining any right of appeal.

Davis dismissed the proposal as a "political document" that had been published in bad faith.

"Well I have to say I thought that document was hardly a legal document," he said in a pooled interview released to journalists.

"It was a political document. What we're about, is building an implementation period, which is to build a bridge to a future where we work well together. And I do not think it was in good faith to publish a document with frankly discourteous language, and actually implying that they could arbitrarily terminate in effect the implementation period. That's not what the aim of this exercise is, it's not in good faith, and we think it's unwise to publish that."

Barnier hit back at Davis denying that his behaviour had been "discourteous" in any way.

"Throughout this negotiations, my attitude has not been in the least discourteous or vindictive. We don't wish to punish the UK. It's totally foreign to my state of my mind," he said.

The latest row follows Barnier's insistence this week that the UK must make its mind up on Brexit.

The EU's chief negotiator told reporters on Monday that the "time has now come to make a choice" on what sort of relationship it wants with the European Union after it leaves the bloc.